The present invention relates to a continuous process for preparing para-alkyl-substituted cyclohexylmethanols, especially 4-isopropylcyclohexylmethanol, by catalytically hydrogenating the corresponding aldehydes or ketones in the presence of hydrogen and of a supported ruthenium catalyst.
4-Alkyl- or -alkenyl-substituted cyclohexylmethanols and the ethers and esters thereof are valuable aroma chemicals which, owing to their lily of the valley-like fragrance, find wide use in the aromatization of everyday or consumable goods of all kinds. A compound which is particularly sought-after in this respect is 4-isopropylcyclohexyl-methanol, which is typically used in the form of a mixture of its cis and trans isomers and is sold under the trade name Mayol® (Firmenich SA, Geneva).
Owing to the constantly rising demand for the aroma chemicals mentioned, specifically for 4-isopropylcyclohexylmethanol, it is becoming ever more difficult for the existing processes for preparing the compounds mentioned to be able to meet this demand. There is therefore a need for a high-performance preparation process for the compounds mentioned which is suitable for preparation on the industrial scale, especially for the preparation of 4-isopropylcyclohexylmethanol.
DE 24 27 609 discloses alicyclic cyclohexylmethanols, and the ethers or esters thereof, which have an isopropyl or isopropenyl radical in the 4 position, and the use thereof as odorants or flavorings. In addition, the document discloses processes for preparing the compounds mentioned by catalytically hydrogenating correspondingly unsaturated starting compounds. By way of example, the preparation of 4-isopropylcyclohexyl-methanol by catalytic hydrogenation of cuminaldehyde in 1,2-dimethoxyethane as a solvent and in the presence of a ruthenium-carbon catalyst with a ruthenium content of 5% is described. The reaction was performed at a pressure of 100 atmospheres and at a temperature of 130° C. and afforded, after fractional distillation of the crude product, 4-isopropylcyclohexylmethanol in the form of a mixture of 70:30 parts by weight of the cis and trans isomers.
EP 0 293 739 relates to a process for preparing 4-isopropylcyclohexylmethanol or alkyl ethers thereof by ring hydrogenation of 4-(1-alkoxy-1-methylethyl)benzaldehydes or of dialkyl acetals thereof in the presence of noble metals of group VIII of the periodic table, for example nickel, palladium, platinum, rhodium and ruthenium. The metals mentioned may be applied to a catalyst support material, for example aluminum oxide or activated carbon, in amounts of from 0.5 to 10% by weight, or else be used in the form of the pure metals or metal compounds. The reactions are performed under hydrogen pressures of from 50 to 350 bar and at temperatures of from 100 to 250° C.
EP 0 992 475 discloses a process for preparing an alcohol by catalytically hydrogenating the corresponding aldehyde, excluding 3-hydroxypropionaldehyde, or ketone in aqueous or organic solution at a temperature of from 20 to 200° C. and an H2 pressure of from 0.5 to 30 MPa using a support-bound ruthenium catalyst, wherein the catalyst used is ruthenium on an oxidic support from the group of TiO2, SiO2, ZrO2, MgO, mixed oxides and silicates, excluding zeolites, with an Ru content of from 0.1 to 20% by weight. The use of TiO2 and SiO2 as support materials achieves high service lives of the catalyst, but ring hydrogenation of aromatic substrates is not described.
EP 1 004 564 relates to a process for preparing hydroxyethylcyclohexanes by catalytically hydrogenating the corresponding hydroxyethylbenzenes by means of ruthenium as a catalyst, which has been treated with a reducing agent before use. The process is performed in an alkane with a boiling point of more than 70° C. as a solvent.
KR 2004072433 discloses a process for preparing 1-cyclohexyl-1-ethanol by continuously hydrogenating 1-methylbenzyl alcohol and/or acetophenone over a silica gel-supported ruthenium catalyst with a ruthenium content of from 1 to 5% by weight, the silica gel support having an “acid activity index” of less than 10%.
EP 1 676 829 relates to a process for continuously catalytically hydrogenating hydrogenatable compounds, specifically aromatic carboxylic acids or derivatives thereof, which are ring-hydrogenated. In the reaction, which is effected over solid catalysts arranged in a fixed bed with a hydrogenous gas, the hydrogenation is performed in at least two hydrogenation units connected in series, and at least one of the two hydrogenation units is operated in loop mode, using, for the hydrogenation units, catalyst volumes which are obtainable by a specific determination process.